The present invention relates to an open-end spinning machine for producing cross-wound bobbins, sometimes referred to as cheeses, wherein the spinning machine is provided with a servicing apparatus equipped for automatic repair of yarn breaks during bobbin winding and for exchanging finished bobbins with empty yarn-winding tubes by utilizing an apparatus for furnishing and removing an auxiliary yarn used for piecing-up the yarn being wound to the replacement tube.
Yarn piecing units operable upon the occurrence of a yarn break to first clean the spinning element and then re-piece the broken yarn using a yarn end taken by suction from the bobbin held in the creel of the spinning station, are known. These piecing units cooperate with special bobbin changing carriages, which assure that finished bobbins will be replaced with empty starter tubes. The starter tubes are transferred from the changing carriage to the creel of the spinning station in a form already containing a certain quantity of yarn which the piecing unit needs for the piecing-up operation.
Service units are also known which can not only repair "normal" yarn breaks but are also equipped to carry out bobbin changing operations utilizing an auxiliary piecing yarn which is inserted and removed to perform a piecing-up operation, such as representatively disclosed in German Patent DE 38 01 965 A, and European Patents EP 0 106 809 B, EP 0 203 508 B, or EP 0 311 987 B.
The piecing unit disclosed in European Patent EP 0 106 806 B, for instance, has an auxiliary yarn transport apparatus with numerous, successively triggered auxiliary yarn transfer elements. The auxiliary yarn drawn from a supply bobbin is first engaged by a yarn clamp disposed on the end of a pivoting lever and then fed by it into the region of a suction nozzle. The suction nozzle transfers the auxiliary piecing yarn to the spinning box of the spinning station. During the transfer of the auxiliary yarn by the pivoting lever, the yarn end is moved past a further suction nozzle, which accordingly aspirates a loop of yarn. This second suction nozzle is pivotable into the region of the bobbin winding mechanism to apply the pieced-up yarn to an empty tube held in the winder creel.
The piecing unit of European Patent EP 0 203 508 B is equipped with an auxiliary yarn transport apparatus having not only two separate suction nozzles but also two individually triggerable yarn clamps. In this arrangement, the auxiliary yarn, which is drawn from the supply bobbin by a yarn delivery mechanism, is engaged at the outlet of the delivery mechanism by a suction nozzle and moved to the region of the bobbin winder, forming a length of the piecing yarn. Next, the auxiliary yarn is positioned by a centering means and aspirated by the yarn locating suction nozzle of the piecing unit. The end of the auxiliary yarn is brought to the region of the spinning box by the first yarn clamping device. Since the first suction nozzle exerts suction continuously, the auxiliary yarn end must be secured by a second yarn clamp during the return of the auxiliary yarn to the spinning rotor since otherwise the yarn end would be aspirated into the first suction nozzle.
European Patent EP 0 311 987 B describes an apparatus with a pneumatic-mechanical auxiliary yarn transport arrangement. An auxiliary yarn drawn from a supply bobbin is first presented by a pneumatic yarn transport device to a mechanical delivery device, which delivers the yarn into the region of the winding mechanism and transfers it to a special suction nozzle of a yarn piecing carriage. Yarn payout after piecing-up then takes place via the mechanical yarn delivery device.
The arrangements described above have various disadvantages. For instance, they are relatively complicated in design and have the risk that the yarn may be lost during the frequent transfers which take place.